[Aaahh!! real monsters!] how come monsters don't eat humans?

in the first episode of the series, a young kid ends up in the monster world and the monsters tell him, "monsters dont eat humans, who knows where they've been?"

but the thing is...throughout the series, we see that monsters ENJOY filth and dont exactly have "healthy" diets. and its known that monsters eat other monsters

so...why DON'T monsters eat humans? do they really care about where a human has "been"?